The dorsal striatum is a brain region crucial for expression of habit and goal-directed behaviors that has recently been linked to addiction. I found that ex vivo or in vivo ethanol exposure and withdrawal causes a long-lasting increase in the activity of the NMDA receptors (NMDARs) that contain NR2B subunits (NR2B- NMDARs) in the dorsomedial striatum (DMS), and that inhibition of NR2B-NMDARs specifically within the DMS attenuates rat operant ethanol self-administration (Wang et al., J Neurosci 2007; Wang et al., J Neurosci 2010). NMDAR-mediated glutamatergic transmission is crucial for synaptic plasticity and learning and memory, as well as for expression of some goal-directed behaviors. The DMS receives a number of different glutamatergic inputs, including corticostriatal (CortSt) and amygdalostriatal (AmygSt), and also contains two groups of principal neurons: dopamine D1 or D2 receptor-expressing medium spiny neurons (D1 or D2 MSNs, respectively). Very little is known about the glutamatergic neurotransmission in ethanol-exposed animals at these different inputs and cell types. Thus, the goal of this application is to elucidate how different afferent glutamatergic inputs in distinct types of neurons within the DMS are altered by excessive ethanol intake, with the long-term goal of determining how such alterations contribute to ethanol-drinking behaviors. My hypothesis, based in part on my preliminary results, is that NR2B-NMDAR-dependent glutamatergic transmission and synaptic plasticity in different striatal circuits (CortSt vs. AmygSt afferents and D1 vs. D2 MSNs) are differentially regulated by excessive ethanol consumption, and that ethanol-related adaptations are predominantly expressed in AmygSt inputs and in D1 MSNs. This hypothesis will be tested by pursuing the following aims: 1) to determine whether excessive ethanol consumption causes a differential increase in the CortSt and AmygSt glutamatergic transmission in the DMS of rats, 2) to determine whether AmygSt transmission facilitates induction of NMDAR-dependent long-term potentiation (LTP) at CortSt synapses in the DMS of naive and ethanol-drinking rats, and 3) to determine whether excessive ethanol consumption alters glutamatergic transmission differentially onto D1 or D2 MSNs of transgenic mice where D1 or D2 MSNs express a fluorescent marker. This application is highly innovative because it applies state-of-the- art approaches including the combination of optogenetics and transgenic mice, allowing us for the first time to determine how ethanol regulates synaptic transmission in different afferent projections to the DMS and different cell types within the DMS, which are important questions that cannot be addressed using conventional methodologies. Knowledge generated from this proposal will greatly facilitate our understanding of the mechanisms by which ethanol-mediated neuroadaptations in glutamatergic transmission are central to promoting pathological ethanol-drinking behaviors.